1. Field of the Invention
The present invention relates to coding in a communications system, and is more particularly related to space-time codes that exploit multiple forms of diversity.
2. Discussion of the Background
Given the multiple-input multiple-output (MIMO) transceivers constant demand for higher system capacity of wireless systems, multiple antenna systems have emerged to increase system bandwidth vis-à-vis single antenna systems. In multiple antenna systems, data is parsed into multiple streams, which are simultaneously transmitted over a corresponding quantity of transmit antennas. At the receiving end, multiple receive antennas are used to reconstruct the original data stream. To combat the detrimental effects of the communication channel, communication engineers are tasked to develop channel codes that optimize system reliability and throughput in a multiple antenna system.
To minimize the effects of the communication channel, which typically is Rayleigh, space-time codes have been garnered significant attention. Rayleigh fading channels introduce noise and attenuation to such an extent that a receiver may not reliably reproduce the transmitted signal without some form of diversity; diversity provides a replica of the transmitted signal. Space-time codes are two dimensional channel codes that exploit spatial transmit diversity, whereby the receiver can reliably detect the transmitted signal. Conventional designs of space-time codes have focused on maximizing spatial diversity in quasi-static fading channels and fast fading channels. However, real communications systems exhibit channel characteristics that are somewhere between quasi-static and fast fading. Accordingly, such conventional space-time codes are not optimized.
Further, other approaches to space-time code design assume that channel state information (CSI) are available at both the transmitter and receiver. Thus, a drawback of such approaches is that the design requires the transmitter and receiver to have knowledge of the CSI, which increases implementation costs because of the need for additional hardware. Moreover, these approaches view the transmit diversity attending the use of space-time codes as a substitute for time diversity; consequently, such space-time codes are not designed to take advantage of other forms of diversity.
Notably, information theoretic studies have shown that spatial diversity provided by multiple transmit and/or receive antennas allows for a significant increase in the capacity of wireless communications systems operated in a flat Rayleigh fading environment [1] [2]. Following this observation, various approaches for exploiting this spatial diversity have been proposed. In one approach, channel coding is performed across the spatial dimension as well as time to benefit from the spatial diversity provided by using multiple transmit antennas [3]. Tarokh et al. coined the term “space-time coding” for this scheme. One potential drawback of this scheme is that the complexity of the maximum likelihood (ML) decoder is exponential in the number of transmit antennas. Another approach, as proposed by Foshini [5], relies upon arranging the transmitted data stream into multiple independent layers and sub-optimal signal processing techniques at the receiver to achieve performance that is asymptotically close to the outage capacity with reasonable complexity. In this approach, no effort is made to optimize the channel coding scheme.
Conventional approaches to space-time coding design have focused primarily on the flat fading channel model. With respect to the treatment of MIMO frequency selective channels, one approach contends the that space-time codes that are designed to achieve a certain diversity order in flat fading channels achieve at least the same diversity order in frequency selective fading channels. Such an approach fails to exploit the spatial and frequency diversity available in the channel.
Based on the foregoing, there is a clear need for improved approaches for providing a system that utilizes space-time codes that can be utilized in a MIMO selective fading channel. There is also a need to design space-time codes that can exploit spatial diversity as well as time diversity. There is also a need to improve system reliability without reducing transmission rate. There is a further need to simplify the receiver design. Therefore, an approach for employing space-time codes that can enhance system reliability and throughput in a multiple antenna system is highly desirable.